The venerable Silicon Valley giant Hewlett-Packard (HP) began making scientific desktop calculators in the 1960s, but the engineers at HP wanted to create something portable, just like a slide rule. Stanford University and MIT alumnus William Hewlett’s shirt pocket measured 5.8 inches long by 3.2 inches wide, which fixed the dimensions for the world’s first pocket-sized scientific calculator, the HP-35, introduced in 1972. Named for its 35 keys, the HP-35 quickly supplanted the slide rule among scientists and engineers at MIT and around the globe.